Dispersion managed optical waveguide

ABSTRACT

A single-mode optical waveguide fiber designed to limit power penalty due to four wave mixing and a method of making the waveguide is disclosed. Variations in properties, e.g., radius or refractive index, of the waveguide fiber core provide a total dispersion which varies along the length of the waveguide. The algebraic sum of products of length times total dispersion is controlled to a pre-selected value for each waveguide fiber which makes up a system link. Proper choice of total dispersion variation magnitude and sub-length results in a system link wherein a signal travels only short distances in waveguide portions having total dispersion near zero. However, the variation of the total dispersion provides a system link which has a pre-selected dispersive effect on the signal over a selected wavelength range. The dispersive effect on the signal can be chosen to be essentially zero. A number of techniques for fabricating DM fiber are also disclosed.

BACKGROUND

The invention is directed to a dispersion managed (DM) single-modeoptical waveguide fiber and a method for making the inventive fiber.

The introduction into the telecommunications network of high poweredlasers, optical amplifiers, multiple channel communications, and higherbit rates has resulted in the exploration of waveguide fiber designswhich can minimize signal degradation due to non-linear waveguideeffects.

Of particular interest is a waveguide design which can substantiallyeliminate four wave mixing. A dilemma arises in the design of awaveguide fiber to minimize four wave mixing while maintainingcharacteristics required for systems which have long spacing betweenregenerators. That is, in order to substantially eliminate four wavemixing, the waveguide fiber should not be operated near its zero oftotal dispersion, because four wave mixing occurs when waveguidedispersion is low, i.e., less than about 0.5 ps/nm-km. On the otherhand, signals, having a wavelength away from the zero of totaldispersion of the waveguide, are degraded because of the presence of thetotal dispersion.

One strategy proposed to overcome this dilemma, is to construct a systemusing cabled waveguide fiber lengths some of which have a positive totaldispersion and some of which have a negative total dispersion. If thelength weighted average of dispersion for all the cable segments isclose to zero, the regenerator spacing can be large. However, the signalessentially never passes through a waveguide length where the dispersionis close to zero, so that four wave mixing is prevented.

The problem with this strategy is that each link between regeneratorsmust be tailored to give the required length weighted average ofdispersion. Maintaining cable dispersion identity from cabling plantthrough to installation is an undesirable added task and source oferror. Further, the need to provide not only the proper dispersion, butalso the proper length of cable having that dispersion, increases thedifficulty of manufacture and leads to increased system cost. A furtherproblem arises when one considers the need for replacement cables.

The present invention overcomes these problems by making each individualfiber a self contained dispersion managed system. A pre-selected, lengthweighted average of total dispersion, i.e., total dispersion product, isdesigned into each waveguide fiber. Thus, the cabled waveguide fibersall have essentially identical dispersion product characteristics, andthere is no need to assign a particular set of cables to a particularpart of the system.

Power penalty due to four wave mixing is essentially eliminated, orreduced to a pre-selected level, while total link dispersion is held toa pre-selected value, which may be a value substantially equal to zero.

Definitions

"Dispersion" refers to pulse broadening and is expressed in ps/nm-km.

"Dispersion Product" refers to dispersion times length and is expressedin ps/nm.

"Phase Mismatch" refers to the difference in phase among the interactingwaves of different center wavelengths which may interact via four wavemixing.

A "Period" is the waveguide fiber length which encompasses a sub-lengthhaving positive dispersion, a sub-length having negative dispersion anda transition length over which the dispersion changes from the positiveto the negative dispersion value.

An "Oscillation Length" is either the positive or negative dispersionsub-length of a period. Where there is no sign associated withoscillation length, the positive and negative oscillation lengths aretaken as equal.

The phase mismatch is proportional to the dispersion product. Also, theaccumulated phase mismatch is proportional to the sum of dispersionproducts. Thus in FIGS. 6-7, the unifying principle of the powerpenalties shown is that each of the power penalties varies as theaccumulated phase mismatch among signals varies. As phase mismatchbecomes larger, the power penalty decreases.

Hence, FIGS. 6-7 may be best understood by regarding them asillustrative of the relation of phase mismatch to total dispersion andoscillation length. An alternative statement is, FIGS. 6-7 show thedependence of power penalty on phase mismatch, where phase mismatch hasbeen expressed in terms of distinct measurable waveguide fibercharacteristics, i.e., total dispersion and oscillation length.

The "Signal Separation" is expressed as the frequency separation ofadjacent multiplexed signals on the waveguide fiber. For example systemscontained in this document, the signal separation is 200 GHz.

SUMMARY OF THE INVENTION

The present invention meets the need for a waveguide fiber lengthwherein total dispersion product is controlled to a pre-selected valuewhich meets a system link requirement. Each waveguide fiber isinterchangeable with any other waveguide fiber designed for that systemlink. Thus, no particular arrangement of cable lengths and totalindividual cable dispersions, in a link, is needed to meet the linkrequirements.

A first aspect of the invention is a single-mode waveguide fiber havinga core region surrounded by a cladding layer. The core region ischaracterized by a refractive index profile, which is the expression ofthe refractive index at each point along the core radius. In order toguide light in the core region, at least a part of the core refractiveindex profile must be greater than the clad index. For mostapplications, the clad has a substantially flat index, although someadvantageous designs have been found wherein the clad layer has anon-instant profile.

In this first aspect, the dispersion of the inventive fiber is made tovary between a range of positive values and a range of negative valuesalong the waveguide length. The dispersion product, expressed as ps/nm,of a particular length, l, is the product (D ps/nm-km * l km). Apositive number of ps/nm will cancel an equal negative number of ps/nm.In general, the dispersion associated with a length l_(i) may vary frompoint to point along l_(i). That is, the dispersion D_(i) lies within apre-determined range of dispersions, but may vary from point to pointalong l_(i). To express the contribution of l_(i) to the dispersionproduct, expressed in ps/nm, l_(i) is made up of segments dl_(i) overwhich the associated total dispersion D_(i) is essentially constant.Then the sum of products dl_(i) * D_(i) characterizes the dispersionproduct contribution of l_(i). Note that, in the limit where dl_(i)approaches zero, the sum of products dl_(i) * D_(i) is simply theintegral of dl_(i) * D_(i) over the length l_(i). If the dispersion isessentially constant over sub-length l_(i), then the sum of products issimply l_(i) * D_(i).

The dispersion of the overall waveguide fiber length is managed bycontrolling the dispersion D_(i) of each segment dl_(i), so that the sumof the products D_(i) * dl_(i) is equal to a pre-selected valueappropriate to a particular system design.

Because this waveguide design reaches its full potential in amultiplexed system, in one embodiment, the sum of products is controlledto a pre-selected value over a wavelength range wherein signals may bemultiplexed.

For high rate systems, having long regenerator spacing, the wavelengthrange in the low attenuation window from about 1525 nm to 1565 nm may beadvantageously chosen. In this case, a preferred embodiment would havethe sum of products targeted at zero over that range of wavelengths.

The D_(i) magnitudes are held above 0.5 ps/nm-km to substantiallyprevent four wave mixing and below about 20 ps/nm-km so that overlylarge swings in the waveguide fiber parameters are not required.

Also the length over which a given total dispersion persists isgenerally greater than about 0.1 km. This lower length limit reduces thepower penalty, (see FIG. 7), and simplifies the manufacturing process.

The period of a DM single-mode waveguide is defined as a first lengthhaving a total dispersion which is within a first range, plus a secondlength having a dispersion which is in a second range, wherein the firstand second ranges are of opposite sign, plus a length over which thedispersion makes a transition between the first and second range. Thesethree lengths need not be adjacent, because the quantity beingcontrolled is the sum of D * dl products over an entire fiber length.However, for ease of process control, the three lengths are generallyarranged as a first length, an adjacent transition length, followed by asecond length adjacent the transition length. To avoid four wave mixingand any associated power penalty over the transition length, it isadvantageous to keep the part of the transition length which has anassociated total dispersion less than about 0.5 ps/nm-km as short aspossible, preferably less than about 500 meters per transition andpreferably no more than 10% of the period.

The dispersion of a waveguide length can be changed by a plurality ofmethods including varying waveguide geometry, waveguide refractiveindex, waveguide refractive index profile, or waveguide composition. Inone embodiment, a core preform, made by any of the processes known tothose of ordinary skill in the art, may be processed to have sections ofreduced diameter. The reduction can be done by any of several methodssuch as heating and stretching one or more sections of the preform or byremoving annular regions of the preform by a mechanical technique suchas grinding and polishing, a chemical technique such as acid etching andpolishing, or an energy bombardment technique such as laser ablation.The resulting core preform is then overcladded, by any of severalmethods, including soot deposition or use of overcladding tubes, to forma draw blank or preform having a uniform, substantially cylindricalouter surface.

In a method similar to that described above, a core preform is processedto have sections of increased diameter. The core preform can be heated,and regions of the preform on opposite sides of the heated region can betraversed toward the heated region to enlarge or bulge the heatedregion. The resulting core preform is then overcladded to form a drawpreform.

When the draw blank is drawn to a fiber of uniform diameter, thewaveguide core radius will be reduced over lengths corresponding to thereduced radius lengths in the core preform. A diameter reduction ofabout 5% to 25% is sufficient to produce the desired positive tonegative dispersion variation. The 25% reduction would be needed only incases where the absolute value of total dispersion is about 20 ps/nm-km.A range of radii variation of 5% to 10% is, in general, sufficient formost applications. As before, the quantity controlled is the sum ofproducts D_(i) * dl_(i) and D_(i) * dl_(j), where D_(i) is a totaldispersion corresponding to a reduced radius r_(i), which lies within afirst range of values, and D_(j) is a total dispersion corresponding toan unreduced radius r_(j) which falls within a second range of values.D_(i) and D_(j) are of different algebraic sign in the operatingwavelength range.

The core diameter variations may also be obtained by reducing segmentsof the diameter of a draw preform or blank, having a substantiallyuniform diameter core portion, and then drawing the preform to awaveguide fiber having a uniform outer diameter. The waveguide core willhave segments of decreased diameter corresponding to draw preformsegments for which the diameter was not reduced.

In another embodiment, the refractive index of the fiber core is variedalong the waveguide length. The refractive index may be changed byirradiating the fiber with radiation in the electromagnetic spectrum, orbombarding the waveguide with sub-atomic particles such as electrons,alpha particles or neutrons. Each particle is characterized by itscorresponding DeBroglie wavelength. One may thus describe particlebombardment as irradiation with a particular DeBroglie wavelength. Apreferred way to cause a variation in core index is to irradiate thewaveguide fiber with ultraviolet light. Due to the nature of the polymercoating materials used to protect the waveguide fiber, the ultravioletirradiation is done after the fiber has passed out of the hot zone ofthe furnace, and before it has received a polymer coating. An indexdifference of as low as 5×10⁻⁶ can serve to limit four wave mixing. Anindex difference greater then about 1.0×10⁻³ is preferred.

The varying refractive index produces a varying total dispersion whichallows the sum of products of D_(i) * dl_(i) and D_(j) * dl_(j) to becontrolled. D_(j) is a total dispersion corresponding to a firstrefractive index range. D_(j) is a total dispersion corresponding to asecond refractive index range. D_(i) and D_(j) are of differentalgebraic sign.

Any of a large number of refractive index profiles provide the requiredflexibility for adjusting waveguide dispersion and thereby varying thetotal dispersion. These are discussed in detail in U.S. Pat. No.4,715,679, Bhagavatula, and applications Ser. No. 08/323,795, Ser. No.08/287,262, and Ser. No. 08/378,780.

A particular set of index profiles, which provides the propertiesnecessary to the inventive waveguide, are those having a core regionwhich includes a central portion, having a refractive index profile, andat least one annular portion, surrounding the central portion having adifferent refractive index profile. Optical fibers having this type ofprofile are referred to as segmented core fibers. The central portionmay have an alpha profile, i.e., one which is described by the equation,n(r)=n_(o) 1-Δ(r/a).sup.α !, where n is refractive index, n_(o) ismaximum refractive index, r is the radial variable, a is fiber coreradius, Δ is % refractive index and α is a number greater than zero. Theterm Δ is defined as (n₁ ² -n₂ ²)/2n₁ ², where n₁ is the maximumrefractive index of a core region and n₂ is the refractive index of theclad glass layer.

In another embodiment, the index profile includes an essentiallyconstant central portion, having a refractive index substantially equalto the clad glass refractive index and the adjacent annular indexportion is a rounded step index. For simplicity and ease of manufacturean index profile having a constant central region and one annularrounded step index portion is preferred. The preferred embodiment ofthis simple index profile has a maximum index % Δ of about 1% and aratio a₁ /a of about 0.5, where a₁ and a are defined in of FIG. 4.

Another aspect of the invention is a single-mode optical waveguidehaving a number of sub-lengths, l_(i), which are made up of segments,dl_(i), which have respective essentially constant associateddispersions, D_(i), where the zeros of dispersion of the D_(i) arewithin a first wavelength range. The remaining sub-lengths, l_(j), whichtogether with the l_(i) and the transition lengths, make up thewaveguide fiber length, are made up of segments dl_(j), which haverespective essentially constant associated dispersions D_(j), and thedispersion zeros of the D_(j) are within a second wavelength range whichis disjoint from the first wavelength range. The common meaning of theterm disjoint is that the two ranges have no values in common. Thelengths and dispersions are chosen so that the algebraic sum of productsdl_(i) * D_(i) and dl_(j) * D_(j) is a pre-selected value over apre-determined wavelength range.

For a dispersion shifted waveguide fiber, a preferred pre-determinedsystem operating range is about 1525 nm to 1565 nm. The first range fordispersion zeros is about 1570 nm to 1680 nm, and the second range isabout 1480 nm to 1535 nm. In such a system the communication signalswill substantially always travel in a waveguide fiber of non-zerodispersion, for example a total dispersion not less than about 0.5ps/nm-km, thereby substantially preventing four wave mixing. However,the overall system total dispersion may be held essentially at zero, sothat little or no signal degradation occurs due to total dispersion,i.e., material plus waveguide dispersion.

Yet another aspect of the invention is a method for making a DMwaveguide fiber. A core preform is prepared having at least onesub-length of reduced diameter relative to the rest of the core preform.A clad layer is applied to provide a draw preform. The draw preform isthen drawn into waveguide fiber having a substantially constant outerdiameter. In keeping with the nominal specifications, the waveguidefiber is substantially free of voids. The core of this waveguide willhave a sub-length of reduced diameter corresponding to the preformsub-length of reduced diameter. The number, longitudinal extent anddepth of the sub-lengths of reduced diameter are chosen so that the sumof the total dispersion times sub-length products equals a pre-selectedvalue over a pre-determined wavelength range. As above, if the totaldispersion varies over a sub-length, the sub-length is made up ofsegments each having an associated, essentially constant, totaldispersion and a characteristic total dispersion times length product.

An alternative process for introducing diameter variations is thereduction of the diameter of at least one sub-length of the draw blank,which is characterized as having a substantially uniform radial coredimension. Drawing the waveguide to a uniform diameter will againproduce a core having diameter fluctuations. In this case the waveguidecore diameter will be smaller in the corresponding draw preformsub-lengths which are not reduced in diameter.

The diameter reduction of either the core preform or draw preform may becarried out using any of several techniques known to those of ordinaryskill in the art. These include the series techniques, grinding andpolishing, etching and polishing or heating and stretching.

Alternatively, core diameter variations can be introduced into the corepreform by heating sections of the core preform and urging portions ofthe core preform on opposite sides of the heated section toward theheated section to cause it to bulge.

As stated above it is advantageous to have reduced diameter waveguidecore lengths greater than about 0.1 km. The amount of the reduction ispreferably in the range of about 5% to 25% of the unreduced corediameter. As stated above a range of 5% to 10% is usually sufficient.

In an embodiment of the method which facilitates manufacturing, thesub-lengths of reduced diameter are evenly spaced along the core or drawpreform to produce in the waveguide pairs of reduced and unreducedwaveguide fiber sub-lengths, which are joined by a transition lengthover which the diameter changes from reduced to unreduced size, or viceversa. If the entire waveguide length is made up of such pairs, theoverall total dispersion can be readily be targeted at zero.

The overall dispersion target may also be a value other than zero. Theentire waveguide fiber length is formed of a number of sub-length pairswhose dispersions essentially cancel and an additional sub-lengthdesigned to have the targeted or pre-selected dispersion times lengthvalues.

Another aspect of the invention is a method of managing dispersion overa length of single-mode waveguide fiber, wherein the refractive indexprofile is increased, over at least one sub-length, by means ofultraviolet radiation so that a difference in total dispersion existsbetween irradiated and un-irradiated sub-lengths. The irradiating stepis advantageously carried out after drawing but before coating of thewaveguide. As mentioned above an index difference greater than about1.0×10⁻³ is preferred.

An irradiation scheme effective to reduce power loss due to four wavemixing is one wherein at least one pulse of ultraviolet light, having anenergy flux of about 100 mJ/cm², delivered over a time in the range ofabout 10 to 20 ns, and, having a spot size of about 1 cm², is incidentupon the fiber. That is, sufficient refractive index variation isproduced in the waveguide fiber to reduce signal power loss due to fourwave mixing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of total dispersion varying along thewaveguide fiber length.

FIG. 2 shows how the zero dispersion of a waveguide fiber may vary tomaintain total dispersion of the waveguide within a pre-selected rangeover a pre-determined wavelength window.

FIG. 3 is an illustration of a waveguide fiber having a bi-modalwaveguide dispersion over a pre-selected wavelength range.

FIG. 4 is a chart of % index delta vs. radius for a simple design whichapproximates a bi-modal waveguide dispersion.

FIG. 5a is a chart illustrating the power penalty vs. input power for asystem comprised of particular waveguide sub-lengths having a low totaldispersion magnitude.

FIG. 5b is a chart illustrating the power penalty vs. input power for asystem comprised of particular waveguide sub-lengths having a highertotal dispersion magnitude.

FIG. 6 is a chart of total dispersion vs. power penalty.

FIG. 7 is a chart of dispersion variation period length vs. powerpenalty.

FIG. 8 is a chart of transition region length vs. power penalty.

FIG. 9 is a schematic of an apparatus for drawing a fiber andirradiating the fiber before applying a protective coating.

FIG. 10a illustrates a longitudinal section of a core preform havinglength sections of reduced diameter.

FIG. 10b illustrates the application of cladding glass particles to acore preform.

FIG. 10c shows undulations that can exist in the surface of the drawblank as a result of diameter variations in the core preform.

FIG. 10d shows a longitudinal section of the core preform of FIG. 10awith an overclad layer having a cylindrical outer surface.

FIG. 11a illustrates a method of periodically reducing the diameter ofsections of a core preform.

FIG. 11b is a temperature profile of the burner flame of FIG. 11a.

FIG. 12 is an enlarged cross-sectional view of the core preform of FIG.11a.

FIG. 13 illustrates the cooling of regions of a core preform adjacentregions that are to be heated and stretched.

FIG. 14 illustrates a device for confining the burner flame to a narrowsection of the core preform.

FIG. 15 shows the heating of a slotted region of a core preform.

FIG. 16 shows the heating of a core preform with a laser beam.

FIG. 17 shows that a laser beam can be used for machining slots in acore preform.

FIG. 18 illustrates the use of heat conducting sleeves.

FIG. 19 illustrates that the core preform can be heated and bulged toperiodically form regions of different diameter.

FIGS. 20-21 illustrate methods of drawing a core preform to periodicallyform regions of different diameter.

FIG. 22 is a graph of an optical time division reflectometry signal froma Dispersion managed fiber showing periodic dips in the signal thatsignify diameter fluctuations in the fiber core.

FIGS. 23a-23c are plots of dispersion vs. wavelength for optical fibershaving different core diameters.

FIG. 24a is a longitudinal section of a draw blank having lengthsections of reduced diameter.

FIG. 24b is a longitudinal section of the essentially constant outerdiameter waveguide which results from drawing the draw blank of FIG.24a.

FIG. 25 shows a method of adding sleeves to a draw blank to form lengthsections of different diameter.

FIG. 26 shows a method of inserting a core preform into a sleevedcladding tube to form a draw blank having length sections of differentdiameter.

FIGS. 27-29 illustrate another method for forming a variable diameterdraw blank from which a variable core diameter dispersion managed fibercan be drawn.

FIGS. 30A and 30B illustrate another method in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a single-mode optical waveguidefiber wherein the total dispersion product, expressed as ps/nm, ismanaged over a waveguide length. The power penalty due to four wavemixing in a wavelength division multiplexed system can thus be largelyavoided and the overall system dispersion can be maintained at apre-selected value. For many long distance, large regenerator spacing,high data rate, multiplexed systems, the desired pre-selected value fortotal dispersion product is essentially zero.

By managing dispersion within each individual fiber, there is no need toselect sets of waveguides which produce a substantially zero dispersionwhen joined together to form a link in a system. That is, because totaldispersion product is managed in the individual waveguide fiber, thecables being installed to form a system are essentially identical inperformance and therefore interchangeable.

The total dispersion, expressed as ps/nm-km, is charted vs. waveguidelength in FIG. 1. The total dispersion is seen to alternate betweenpositive values 2 and negative values 4. Whereas FIG. 1 illustrates aplurality of sublengths exhibiting negative dispersion and a pluralityof sublengths exhibiting positive dispersion, only one negativedispersion sublength and one positive dispersion sublength are required.The spread in total dispersion values indicated by line 6 illustratesthat total dispersion varies with the wavelength of light propagated.The horizontal lines of the spread 6 represent total dispersion for aparticular light wavelength. In general, the length of waveguide 8,characterized by a particular total dispersion, is greater than about0.1 km. There is essentially no upper limit on length 8 except one whichmay be inferred from the requirement that the sum of products, length xcorresponding total dispersion, is equal to a pre-selected value.

The chart of total dispersion vs. wavelength shown in FIG. 2 serves toillustrate design considerations for a DM single-mode waveguide fiber.Lines 10, 12, 14 and 16 represent total dispersion for four individualwaveguide fibers. Over the narrow wavelength range considered for eachwaveguide, i.e., about 30 nm, the dispersion may be approximated by astraight line as shown. The wavelength range in which multiplexing is tobe done is the range from 26 to 28. Any waveguide segment which has azero dispersion wavelength in the range of 18 to 20 may be combined witha waveguide segment having a zero dispersion wavelength in the range 22to 24, to yield a waveguide having a pre-selected total dispersion overthe operating window 26 to 28.

EXAMPLE BASED ON FIG. 2

Take the operating window to be 1540 n to 1565 nm. Assume that thesingle-mode waveguide fiber has a dispersion slope of about 0.08 ps/nm²-km. Let line 30 be the 0.5 ps/nm-km value and line 32 the 4 ps/nm-kmvalue. Apply the condition that the total dispersion within theoperating window must be in the range of about 0.5 to 4 ps/nm-km.

A simple straight line calculation then yields zero dispersionwavelength range, 18 to 20, of 1515 nm to 1534 nm. A similar calculationyields a zero dispersion wavelength range, 22 to 24, of 1570 nm to 1590nm. Algebraic addition of the total dispersion of waveguide fibersegments having dispersion zero within the stated ranges will yield atotal dispersion between 0.5 and 4 ps/nm-km.

As described above the means for shifting zero dispersion wavelengthinclude varying radius or %Δ along the core preform or draw preformlength.

Model calculations have shown that bimodal waveguide dispersion curveslike curve 34 in FIG. 3 are possible. Curve 34 corresponds to therefractive index profile represented by solid line 38 illustrated inFIG. 4. Fiber designs exhibiting bimodal dispersion are disclosed inU.S. patent application Ser. No. 08/287,262. Curve 34 includesrelatively flat regions 34a and 34b and a relatively steep transitionregion 34c. Line 35 of FIG. 3 signifies that the low λ₀ wavelengthregion 34a and the high λ₀ wavelength region 34b can be well controlledand at the same time widely separated. Where line 37 crosses curve 36 orcurve 34 represents the average between the high and the low waveguidedispersions. For waveguide fibers having a bi-modal waveguide dispersionas shown in curve 34 of FIG. 3, only small changes in cut-offwavelength, or the proportional quantity, core radius, are required toproduce the relatively large changes in zero dispersion wavelength asindicated in the example based on FIG. 2. Fibers that have dispersioncharacteristics represented by curve 34 also have the followingadvantage. In DM fibers that have alternate regions of large and smallcore diameter, the manufacturing process may result in slight deviationsin diameter from design diameter. To illustrate this point, assume thatthe operating points for the fiber regions of different core diameterare D₁ and D₂ in FIG. 3. If D₁ and D₂ are located along the relativelyflat regions of the bimodal curve, small fluctuations in core diameteror cutoff wavelength around D₁ and D₂ will not significantly alter theλ₀ value of the small core diameter regions nor the λ₀ value of thelarge core diameter regions.

An ideal profile might be that represented by curve 38 of FIG. 4, theinner and outer radii of the core ring being represented by a₁ and a,respectively. However, certain manufacturing techniques may result indopant diffusion causing the profile to deviate from the ideal profileto a profile such as that represented by dashed line curve 40. The innerradius of the core ring of profile 40 is smaller than a, and the outerradius is larger than a. This would make the waveguide dispersion curveless steep as shown in dashed line curve 36 of FIG. 3.

One may choose to use a profile which has a waveguide dispersioncharacteristic such as 36 in FIG. 3 to simplify the manufacturingprocess. This is clearly a weighing of cost compared to benefit. Theparticular telecommunication application will dictate whether a morecomplex profile is warranted. However, to best manage the totaldispersion product, a waveguide fiber which has a sharp transitionregion separating waveguide dispersions of different levels ispreferred. The required shifting between two disjoint wavelength rangesof dispersion zero may then be accomplished with smaller changes inradius, as low as 5% to 10% radius change, for profiles having thebi-modal waveguide dispersion. Thus the management of the sign change oftotal dispersion is facilitated. Also the distance in the waveguide thatthe signal travels, over which the total dispersion is less than about0.5 ps/nm-km, is held to a minimum.

The design of the DM fiber depends strongly on the details of thetelecommunication system as can be seen in FIG. 5a and 5b which showpower penalty charted vs. input power for a 120 km link having 8channels, wherein the frequency separation of channels is 200 GHz. Inthis case the power penalty is that due primarily to four wave mixing.Curve 62 in FIG. 5a rises steeply to a penalty near 1 dB for an inputpower of about 10 dBm. The penalty is about 0.6 dB for an input power of10 dBm (curve 64). For both curves the magnitude of the total dispersionis about 0.5 ps/nm-km. However, for the steeper curve 62 the sub-lengthfor total dispersion of a given sign is 10 km. The correspondingsub-length of the dispersion in curve 64 is 60 km. The extra penaltyresults from the additional transitions through zero dispersion for theshorter, 10 km sub-length case. An alternative statement is for the 10km case, the phase separation of the signals, which is proportional tothe oscillation sub-length, is not large enough to substantially preventfour wave mixing.

However, magnitude of the total dispersion also has an impact upon phaseseparation and thus upon power penalty. Curve 66 in FIG. 5b shows thepower penalty for a system identical to that shown in FIG. 5a, exceptthat the sub-length is shorter, about 1 km, but the total dispersionmagnitude is 1.5 ps/nm-km. Causing the waveguide total dispersion tomake wider positive to negative swings reduces power penaltysignificantly, from 0.6 dB to less than 0.2 dB. The penalty differenceof about 0.4 dB/120 km is large enough to be the difference between afunctional and non-functional link, especially for long unregeneratedlinks of 500 km or more.

FIG. 3 is interpreted in essentially the same manner as FIGS. 5a an 5b.Curve 68 shows power penalty charted vs. total dispersion magnitude. Thesub-length of the waveguide is chosen as about 1 km because the lengthof the shortest cables in general use is about 2 km. Again there are 8channels having a frequency separation of 200 GHz, a total length of 120km, and the input power is 10 dBm. Again the power penalty rises steeplywhen total dispersion magnitude falls below about 1.5 ps/nm-km.

System design is shown from another viewpoint in FIG. 7. in this case,the dispersion magnitude is fixed at 1.5 ps/nm-km. Curve 70 representspower penalty vs. sub-length magnitude for a system having 8 channelswith 200 GHz frequency separation and 10 dBm input power. The length ischosen to be 60 dispersion sub-lengths and the sub-length is allowed tovary. Lower power penalties result when the sub-length is above 2 km.But with the relatively large total dispersion magnitude, little isgained by lengthening the sub-length beyond 2 km. Note the generallylower four wave mixing penalty paid when the number of channels used isreduced to 4 as shown by curve 72. This latter phenomenon is a directresult of the dependence of phase separation on the fibercharacteristics noted above and in the "Definitions" section.

Another design consideration is the sharpness of the transition lengthover which the total dispersion changes sign. Here also, the signalphase separation is affected by the transition length. Thus, a shallowtransition would cause the signal to travel a waveguide region of nearzero total dispersion, and this adversely impacts power penalty causedby four wave mixing.

EXAMPLE BASED ON FIG. 8

In this case, the input power is again 10 dBm. Four channels are usedhaving a frequency separation of 200 GHz. The magnitude of totaldispersion is 1.5 ps/nm-km and the oscillation length of the totaldispersion is taken to be 2 km. The chart of power penalty vs.transition length, shown as curve 74 in FIG. 8, shows that shortertransition lengths are preferred. Also, the number of transitions shouldbe a minimum, in the framework of other design considerations and costbenefit studies. Because some systems may allow a power penalty of about1 dB, a maximum tolerable transition length is about 500 meters, asshown in FIG. 8.

FIBER FABRICATION TECHNIQUES

The period of the optical fiber can be chosen to be a few tens of metersto hundreds of meters so that over any reasonable link length, theaverage dispersion is quite small over the operating wavelength window.

If the transition regions between the regions of higher and lowerdispersion are too long, the dispersion in the central portions of thetransition regions will be near zero for some finite length of fiber.This will result in some power penalty due to four wave mixing. Thelonger the transition regions are, the higher the power penalty. Thetransition regions should therefore be sufficiently sharp that the fiberpower penalty does not cause the total system power penalty to exceedthe allocated power penalty budget. Moreover, the process should not beone that itself induces an excess loss that is unrelated to four wavemixing. Also, the process should be simple and be sufficiently flexiblethat it can be implemented with a variety of fiber designs andmaterials.

The disclosed techniques include: (a) modification of core refractiveindex during draw, (b) fabrication of a draw preform having a variablediameter core and a constant diameter cladding or a substantiallyconstant diameter cladding, and (c) fabrication of a draw preform havinga constant diameter core and a variable diameter cladding. Moreover,combinations of these techniques could be employed.

(a) Modification of Core Refractive Index During Draw

A schematic of waveguide fiber draw apparatus is shown in FIG. 9. As iswell known in the art, draw blank 77 is heated in furnace 78, and drawninto waveguide fiber 79. Waveguide 79 is given a protective coating atcoating apparatus 80 and wound onto a take up reel 81. The noveladdition to this apparatus is a high power, well focused source ofultraviolet radiation 76, directed at the waveguide fiber after drawingbut before coating. Ultraviolet light is known to have the effect ofraising refractive index in a doped region of the waveguide. Thusultraviolet light will act to increase the refractive index of the coreregion. The ultraviolet source is turned off and on periodically toinduce variation in refractive index along the waveguide fiber length.The variations must be large enough to produce variation in totaldispersion sufficient to reduce power penalty due to four wave mixing.It has been found that at least one pulse of 10 to 20 ns duration havingan energy flux of about 100 mJ/cM² is sufficient to produce somebeneficial effect. An excimer laser, operating at about 248 nm, is anexcellent source of high intensity, well focused ultraviolet radiation.

(b) Modify Core Preform Diameter; Then Overclad

A core preform is a preform that, after being provided with claddingglass, can be drawn into an optical fiber. An economical technique forforming high quality core preforms includes the steps of (a) forming aporous glass preform by a glass particle deposition process, (b)consolidating the porous preform to form a consolidated preform, (c)closing the axial aperture in the consolidated preform if such anaperture exists, and (d) optionally stretching the preform beforeapplying the cladding (the aperture closing step is advantageouslyperformed during this stretching step). See U.S. Pat. No. 4,486,212,which is incorporated herein by reference. As disclosed in U.S. Pat. No.4,486,212, the core preform may comprise a central region of core glasssurrounded by a thin layer of cladding glass. Alternatively, the corepreform may consist entirely of core glass, as shown in FIG. 10a.

FIG. 10a shows a glass core preform 82 of diameter 83. At intervalsalong the core preform length, the diameter is reduced as shown bydiameter 84. The length of the reduced area is shown by line 85.Diameter 84 may be reduced relative to diameter 83 by 5% to 25% to yieldthe desired variation in core diameter. The length 85 is adjusted toyield in the resultant optical fiber the desired sub-length of reducedcore diameter. The diameter of the core preform may be reduced by any ofseveral methods known to those skilled in the art, including grinding,etching, laser ablation and heating and stretching. A polishing step maybe used after grinding or etching to insure a uniform and cleaninterface between core preform and cladding layer.

If reduced diameter regions are formed in the core preform by machiningor etching into the core, then it is preferred that the core have a stepindex profile. That portion of the core that is removed will havesubstantially the same refractive index as the remaining portion.

Core preform 82 of FIG. 10a is rotated (arrow 82a) and translated alongits longitudinal axis with respect to burner 86 which directs a stream87 of cladding glass particles thereon to build up a layer 88 ofcladding glass particles. The resultant coated core preform is insertedinto a consolidation furnace where coating 88 is dried and sintered toform a draw blank having a dense glass cladding layer. If the length 85of the core preform slots is at least a few millimeters, the claddingparticles 87 will fill the reduced diameter regions. The outer surfaceof the draw preform will therefore be slightly nonuniform. The diametervariations in the draw blank will tend to be small because the amount ofthe core diameter reduction is small.

The overclad soot 88 of the coated core preform or the consolidatedglass draw preform may be smoothed by ordinary techniques to insure acylindrical shape for the draw preform. Since the core preform isfragile, the diameter modification step is preferably performed on theconsolidated preform. FIG. 10c shows a consolidated preform 89 havingdiameter variations. The thickness of these variations is exagerated forthe sake of clarity. Preform 89 can be rotated about its centrallongitudinal axis, and the regions of larger radius can be removed bymachining, etching, or the like, so that surface regions 90 have thesame diameter as the remainder of the preform. As shown in FIG. 10d, theresultant draw preform has a substantially uniform outer diameter 91.The modified core preform 82 is shown encased in cladding material 92.

Also, the overcladding method may readily be modified by one skilled inthe art to insure a uniform diameter draw preform. For example, the rateof traverse of burner 86 with respect to core preform 82 can be variedin accordance with the particular sublength of core preform that isbeing built up by stream 87. The burner traverse is slower as itdeposits particles on the narrow diameter core regions than when itdeposits on the large diameter core regions. The buildup of claddingglass particles over the narrow and wide core preform regions can besuch that the diameter of the of the draw blank produced byconsolidating the particles is essentially constant.

When the draw preform of FIG. 10d is drawn to a uniform diameterwaveguide fiber, the reduced diameter core preform portions becomereduced diameter core portions of the fiber. If a draw blank has regionsof larger diameter, such as those shown in FIG. 10c, the core of thefiber sublength drawn from that larger diameter region will have adiameter that is smaller than desired. If the larger diameter regions ofthe draw blank are not removed as described above, the diminishing ofthe fiber core diameter in those regions of the resultant fiber willhave to be considered when designing the DM fiber.

In the embodiment shown in FIGS. 11a and 12, a core preform 93 of anyappropriate fiber core design is mounted in a lathe 96, 98 (horizontalor vertical) or other appropriate equipment and heated with a verynarrow flame 102 from burner 100. The heat from the burner is preferablyaxially localized to no more than a few millimeters along the corepreform. The temperature and heat capacity of the flame must besufficient to locally soften the core preform glass. As shown in FIG.12, core preform 93 includes a core region 95 surrounded by a layer 94of cladding glass. If the fiber is a silica based fiber that is tooperate in the 1500-1600 nm window, core preform 93 would be one that issuitable for forming a dispersion shifted fiber having zero dispersionin that range. As the glass reaches its softening point, the corepreform is pulled to reduce the diameter in the heated region. Thisproduces the narrow diameter regions 108. The pulling step is normallyperformed while the flame is directed at the region being pulled. If ahorizontal lathe is used, rotation of the core preform would helpprevent any distortion of the core preform during the pulling operation.The heating and pulling steps are performed at the required intervalsalong the length of the core preform. The stretching is preferably donesuch that the lengths L₁ and L₂ of the core preform at the two diametersare approximately equal. The burner is then moved at a rapid rate to thenext region that is to be softened and stretched.

FIG. 11a shows one type of burner design that provides a very localizedheating condition that results in sharp transitions between the corepreform regions of different diameters. The face of burner 100 has acircular array of combustion gas orifices that provides a flame 102, anda circular array of outer shield gas orifices that provide a cylindricalstream of cooling gas. Outer shield stream 103 confines and focusesflame 102 and at the same time provides convection cooling on the corepreform outside of the localized heating zone. The flame should have asuniform a temperature as possible across the hot zone with as sharp atemperature gradient as possible at the edge of the hot zone. This willresult in a relatively short transition length L_(T). Temperatureprofile 106 of FIG. 11b represents a preferred profile across regionA--A of flame 102.

Other types of burners or burner combinations can also be used for thispurpose. For example, a focusing burner such as that disclosed in U.S.Pat. No. 3,565,345 contains slanted nozzle openings to direct streams ofcombustable gas to a common point. A ring burner capable of providing aflame confined by a focusing outer shield gas stream could surround thecore preform and simultaneously heat an entire circumferential region ofthe preform. A ribbon burner having one or more linear arrays ofcombustion gas orifices and optionally containing linear arrays ofshield gas orifices could be employed, the array of combustion gasorifices being disposed perpendicular to the longitudinal axis of thecore preform.

EXAMPLES BASED ON FIGS. 11a AND 12

A core preform 93 having a cladding diameter of about 7 mm and a corediameter of about 5 mm is reduced in diameter to about 6.5 mm atintervals along the core preform length. The length L₁ (FIG. 12) ischosen to be 2 mm. The core preform is overclad as shown in FIG. 10b andconsolidated to form a draw preform having a final diameter of about 50mm.

When the draw preform is drawn into a waveguide having a uniform outerdiameter of about 125 μm, the length L₁ becomes about 320 meters,assuming that the mass of the 2 mm segment is conserved and that thecore radius variation, expressed as % difference, is about 14%. Thisexample assumes that the refractive index profile of the core is of thetype disclosed in U.S. Pat. No. 4,715,679 and U.S. patent applicationsSer. No. 08/323,795, Ser. No. 08/287,262, and Ser. No. 08/378,780,whereby a core diameter of about 16 μm will result in single-modepropagation.

If the draw preform diameter is about 100 mm, under the same corepreform and draw conditions, length 85 becomes 1280 meters and the coreradius variation is unchanged.

FIG. 13 shows that additional localization of the heat can be achievedby directing external focused cooling jets 122 of air, nitrogen, heliumor the like from sources 120 onto that portion of core preform 116adjacent the region that is to be heated by flame 118. By "externalcooling jets" is meant jets that originate from a source other than theburner face. These jets could flow from orifices formed in a plate atthe end of the coolant gas delivery tube. Sources 120 could bepositioned 180° with respect to the burner, as shown, or they could bepositioned 90° or any other suitable orientation with respect to theburner that ensures that the jets do not interfere with the heating ofthe core preform.

Another technique for sharpening the temperature profile of the flame isillustrated in FIG. 14. Shields or baffles 128 deflect portions 132 ofthe flame and allow only central region 130 of the flame to heat corepreform 126. The baffles can consist of cooled plates of metal, ceramicor carbon. Periodic slots 138 are initially formed in core preform 136of FIG. 15 by etching, grinding, laser ablation or the like to reducethe mass of the core preform within the slots. The flame heats up theseslotted regions more rapidly and preferentially as compared to theadjacent large diameter regions. When a slotted region is heated andstretched to form narrow diameter region 142, it therefore forms arelatively sharp transition region.

FIG. 16 illustrates the use of a laser 148, such as a CO₂ laser, Ofsufficient power to provide a beam 147 for locally heating and softeningcore preform 146 so that it can be stretched as described above.

The power and/or duration of the laser beam 154 (FIG. 17) can besufficient to ablate material from a core preform 151 to form narrowdiameter regions 156. Examples of lasers suitable for this purpose areCO₂ and Excimer lasers. Since the regions formed by laser ablation arerelatively smooth and since the laser beam can cut through the claddingportion 153 of the core preform and into the core region 152, nostretching step is required to form the different diameter core regions.

FIG. 18 illustrates the use of heat conducting sleeves 162, which areoptionally provided with means for conducting a coolant medium on orwithin the surface thereof. Sleeves 162 localize the region of corepreform 160 that is sufficiently heated to permit stretching. The corepreform is therefore provided with a relatively sharp diametertransition when it is stretched.

In the embodiment of FIG. 19, the core preform 165 is inserted into aflame working lathe as described above. Spaced regions are heated byflame 167. Instead of pulling the core preform, opposite sides of theheated region are traversed toward the heated region, thereby increasingthe diameter in the heated region with sharp bulges 166.

A multi-diameter core preform can be formed in a draw furnace (FIG. 20)which includes resistance heater 171. Preform feed apparatus 175 andmotor-driven tractors 176 advance into the hot zone generated by heater171 that portion of core preform 170 that is to be stretched. After theglass is soft enough to be stretched, tractors 174 and/or the preformfeed apparatus 175 pull the engaged portion of the core preform awayfrom the heated region to stretch it, thereby forming small diameterregion 172. Means 175 and 176 then traverse the core preform through thehot zone generated by heater 171 until the next region to be stretchedis positioned in it. This stretching of the core preform is repeated atspaced regions along its length such that unstretched large diameterregions 173 are positioned between the small diameter regions 172.Various techniques for drawing multi diameter rods are disclosed in U.S.Pat. No. 4,704,151, which is incorporated herein by reference. Forexample, tractors 176 could be spring loaded, as indicated by arrows 177to ensure that they are in constant contact with the multi-diameter corepreform 178.

It is noted that the heated regions of the core preform of FIG. 20 couldbe made to enlarge rather than reduce in diameter by merely causing theredraw tractors 174 and/or the delivery apparatus 175 advance thepreform toward the heated region.

Referring to FIG. 21, wherein furnace elements similar to those of FIG.20 are indicated by the same reference numerals, a multidiameter corepreform 182 is drawn from the consolidated preform 181 in a draw furnacewhich includes feed apparatus 175, heater 171 and tractors 176.Consolidated core preform 181 is of the type formed by the method of theaforementioned U.S. Pat. No. 4,486,212, whereby a longitudinal apertureextends therethrough. Preform 181 is fed to the hot zone at a constantrate by feed means 175. The preform aperture is evacuated by affixing tothe and of the preform a vacuum fixture 184 that is connected to avacuum source V. The tractors pull at a first draw rate to form thelarge diameter regions 185 and at a higher rate to form the narrowdiameter regions 186. As the diameter of preform 181 decreases to formeither the large or the small diameter regions of core preform 182, theevacuated aperture collapses. Some of the above mentioned approaches canbe used in conjunction with others of the above mentioned approaches toimprove the control on the process and to decrease the length L_(T) ofthe diameter transition regions.

After the core preform is stretched or expanded in accordance with anyof the methods described above, it can be overclad using normalprocedures. If the core preform diameter and overclad deposition weightare properly adjusted, a substantially cylindrical draw blank can beformed during the consolidation process.

EXAMPLE BASED ON TECHNIQUE OF FIGS. 11a AND 12

A preform was formed by a glass particle deposition process of the typedisclosed in U.S. Pat. No. 4,486,212. The preform was stretched to forma core preform having an outside diameter (OD) of 7 mm and a corediameter of 4.55 mm. The core refractive index profile was that of astandard dispersion shifted fiber and was similar to that disclosed inU.S. patent application Ser. No. 08/323,795. The profile included acentral region of GeO₂ -doped SiO₂ (where the GeO₂ content decreasedsubstantially linearly with radius) surrounded by a layer ofsubstantially pure SiO₂ which was in turn surrounded by a layer of GeO₂doped SiO₂. The peak GeO₂ concentration (at the center of the core) wasabout 20 wt. %. The cladding was formed of pure silica. The diameter ofsections of the core preform was reduced by the technique disclosed inconnection with FIG. 11a. The heat source was a small burner known asType 3A blowpipe torch having a 1 mm nozzle; it is made by VerifloCorporation of Richmond, Calif. The fuel was hydrogen and oxygen. Alength of the core preform was not stretched so that a constant corediameter reference fiber could be drawn. The resultant core preform wasoverclad with silica particles. The coating of cladding glass particleswas consolidated, and separate fibers were drawn from the referencesection and from the section having diameter variations.

The optical time division reflectometry (OTDR) signal from the DM fiber(FIG. 22) shows periodic dips in the signal, thereby signifying diameterfluctuations. The trace shows uniform sections with reasonably sharptransitions. The length of a period is about 600 meters. The fiber drawnfrom the reference section of the draw blank had an OD of 125 μm and alength of 2.0 km. As shown in FIG. 23(a) the reference fiber exhibitedzero dispersion at 1500 nm.

A DM fiber drawn from the flame-stretched portion of the draw blank toan OD of 130 μm and a length of 3.6 km has a zero dispersion wavelengthof 1525 nm as shown in FIG. 23(b). A DM fiber drawn from theflame-stretched portion of the draw blank to an OID of 120 μm and alength of 4.0 km has a zero dispersion wavelength of 1544.5 nm as shownin FIG. 23(c). Thus, a DM fiber drawn from the flame-stretched portionof the draw blank to an OD of 125 μm would exhibit a zero dispersionwavelength of 1535 nm. This indicates an average shift in zerodispersion wavelength of about 35 nm (for a 125 μm fiber) compared tothe reference fiber. It can therefore be concluded that the dispersionis fluctuating between 1500 nm and about 1570 nm to give an average of1535 nm. Similar variations in cutoff wavelengths have also beenobserved. These results illustrate that DM fibers having high and lowvalues of λ₀ in the 1500 nm to 1600 nm range have been fabricated.

Back reflection data indicates that even with the diameter variations inthe fiber, the back reflection is only slightly higher than that for afiber having a constant diameter core.

(c) Form Draw Blank Having Variable Cladding Diameter

An alternative method for introducing the core radius variation isillustrated in FIGS. 24a and 24b. In this case the draw blank 187 isreduced in diameter at intervals along its length by grinding, lasermachining, etching or the like. The resultant draw blank has preselectedsublengths 189 having relatively large diameter and preselected smallerdiameter sublengths 190. Note that the diameter of core region 188 isuniform. The subsequent drawing of draw blank 187 to a fiber 192 havinga uniform outside diameter 193 transfers the diameter variation from theouter surface of the draw preform to the fiber core 194. As described inconjunction with FIGS. 10a and 10b, knowing the lengths and diameters ofthe core and cladding of the large and small diameter sections of thedraw blank will permit the corresponding lengths and diameters of theresultant optical fiber to be calculated by asserting conservation ofmass of the appropriate draw blank segment. As shown in FIGS. 24b, fiber192 includes sub-lengths, l_(i), of reduced core diameter correspondingto the draw preform sub-lengths of larger diameter, the remainder offiber 192 comprising sub-lengths, l_(j) of larger core diameter. Thereduced core diameter sub-lengths are made up of segments dl_(i), havingan associated total dispersion product dl_(i) * D_(i), and the largerdiameter sub-lengths are made up of segments dl_(j), having anassociated total dispersion product dl_(j) * D_(j). The algebraic sum ofproducts dl_(i) * D_(i) and dl_(j) * D_(j) is equal to a pre-selectedvalue, over a predetermined wavelength range R.

In the embodiment shown in FIG. 25, a draw blank 201 includes a constantdiameter core 202 surrounded by a constant diameter cladding layer 203.The core diameter is sufficient to provide a fiber section having agiven zero dispersion wavelength. Sleeves 205 of cladding glass arefused to draw blank 201 at periodic positions along its length to form amodified draw blank from which a DM fiber can be drawn. As shown in FIG.24(b), the resultant optical fiber will have a constant outsidediameter, and the core diameter will vary in accordance with theperiodicity of sleeves 203. The fiber drawn from the region of the blanksurrounded by a sleeve will have a smaller diameter core than theregions of the blank having no sleeve. The dispersion of the narrowdiameter portion of the DM fiber will exhibit a zero dispersionwavelength different from the given zero dispersion wavelength.

In accordance with the method of FIG. 26 there is initially provided aperiodically-shaped cladding glass sleeve 209 having small diameterportions 210 and large diameter portions 211 and a bore 212 extendingalong the central longitudinal axis. Sleeve 209 can be formed bytechniques such as machining a cylindrically-shaped glass tube or byfusing glass sleeves onto a cylindrically shaped tube. A core preform213 preferably including a core region 214 and a thin cladding layer 215is formed by any suitable technique. As indicated by arrow 216, corepreform 213 is inserted into bore 212. As described above, the resultantdraw blank is drawn into an optical fiber having a constant outsidediameter, and a core diameter that varies in accordance with theperiodicity of sleeves 211.

The methods of FIGS. 25 and 26 separate the core preform fabricationprocess from the process of shaping that part of the draw blank thatintroduces the periodic aspect of the draw blank. The process of makingdispersion shifted fiber of the type disclosed in U.S. Pat. No.4,715,679 and U.S. patent applications Ser. No. 08/323,795, Ser. No.08/287,262, and Ser. No. 08/378,780 includes a tuning step directed bymeasurements on the core preform. Tapering or etching the core preformcould complicate that tuning process. By using independently preparedsleeves, the process of making the draw blank will have minimal impacton the core preform tuning process.

The methods of FIGS. 25 and 26 should result in very sharp transitionsbetween the two regions of different dispersion; as stated above, thisis a desirable characteristic of DM fiber.

FIGS. 27-29 show a core preform 218 having a core region 219 and acladding 220. Annular slots 221 are formed in the surface of preform 218by grinding, laser machining or the like an accordance with this method,slots 221 should not extend into the core region. Burner 226 directs astream 225 of cladding glass particles onto preform 218 to build up aporous glass layer 227.

An enlarged view of a single slot is shown in FIG. 28. The maximumlength 222 of 221 is about 1 to 2 mm. The flow conditions of stream 225impinging upon a surface having such a short annular slot results in alow density glass particle buildup within the slot. The density of thedots in FIG. 28 represents the density of the deposited glass particles.As the depth 223 of the slot increases, the density of the buildupdecreases. The density of buildup is also affected by the composition ofthe glass particles. Softer glass particles result in the formation of adenser buildup. Thus, particles of pure silica, which is a very highviscosity glass, form a buildup of very low density in slots 221.Indeed, even voids can form within slots 221 depending on particledeposition conditions. After layer 227 has been built up to a thicknesssufficient to form the cladding of a single-mode optical fiber, theresultant preform is consolidated (dried and sintered). This step isconventionally carried out in an atmosphere of helium and a smallpercentage of chlorine to dehydrate the glass particles. The preformcould be soaked in pure helium prior to the sintering process to degasany voids during the later high temperature sintering step. The sintereddraw blank has sublengths of reduced diameter where the glass particleshad been deposited over the slots. When the draw blank is drawn into asingle-mode fiber 241, the core includes smaller diameter regions 242and larger diameter regions 243 which correspond to those regions of thedraw blank where the slots had been.

A method that produces very short transition regions is illustrated inFIGS. 30a and 30b. Two different core preforms are made by a method suchas that disclosed in U.S. Pat. No. 4,486,212. Both core preforms havecore refractive index profiles of the type that yield dispersion shiftedfibers. The first core preform is such that if it were provided withcladding and drawn into a single-mode fiber having a 125 μm OD, it wouldexhibit zero dispersion at 1520 nm. The second preform is such that ifit were similarly formed into a 125 μm OD single-mode fiber, its zerodispersion wavelength would be 1570 nm. Both core preforms are stretchedto a diameter slightly less than 7.5 mm. The first stretched preform iscut into tablets 250, and the second stretched preform is cut intotablets 252 that preferably have the same length as tablets 250. Thetablets are made by the simple score and snap method.

A short length 264 of capillary tubing is fused to one end of a silicatube 266 having an inside diameter (ID) of 7.5 mm and an O.D. of 9 mm.Tube 266 is overclad with silica particles by the method of FIG. 10b toform a porous silica coating 268. Layer 268 is built up to a sufficientOD that the resultant preform can be consolidated and drawn into a 125μm OD single-mode fiber. Tablets 250 and 252 are alternately insertedinto tube 266. Tube 270 is fused to the end of tube 266 opposite tube264. Tube 270 is part of a ball joint type gas feed system of the typedisclosed in U.S. Pat. No. 5,180,410.

The resultant assembly 272 is suspended in a consolidation furnace.While assembly 272 is rotated at 1 rpm, it is lowered into consolidationfurnace muffle 274 at a rate of 5 mm per minute. A gas mixturecomprising 50 sccm chlorine and 40 slpm helium flows upwardly throughthe muffle. Chlorine (arrow 276 flows downwardly around tablets 250 and252 and exhausts through tube 264. A centerline flow of 0.3 slpmchlorine is suitable. The maximum temperature in the consolidationfurnace is about 1450° C. As assembly 272 moves downwardly into thefurnace, it is subjected to a sufficienly high temperature that thecenterline chlorine flow chemically cleans the adjacent surfaces oftablets 250 and 252 and tube 266. As assembly 272 moves further into thefurnace muffle, tube 264 fuses and cuts off the centerline chlorineflow. A valve is then switched to pull a vacuum within tube 266. Asassembly 272 continues its movement into the furnace muffle, first itstip and then the remainder of the assembly is subjected to the maximumfurnace temperature which is sufficient to consolidate coating 268.During consolidation of coating 268, tube 266 is forced inwardly againsttablets 250 and 252, and the contacting surfaces become fused.

The fused assembly is removed from the consolidation furnace and isdrawn to form a dispersion managed optical fiber having an OD of 125 μm.

Single-mode dispersion managed optical fibers made by the foregoingprocess have been drawn without upsets; attenuation has typically been21 dB/km. The two different types of tablets that were employed in thefiber making process combined to provide a zero dispersion wavelength of1550 nm. The oscillation lengths and the period are controlled by thelengths of the core preform tablets. Fibers having oscillation lengthsof 1.2 to 2.5 km have been drawn.

Thus a waveguide fiber and methods for making a waveguide which meetsthe requirements of a high data rate, high power, multiplexed systemhave been disclosed and described. Although particular embodiments ofthe invention have been discussed in detail, the invention isnevertheless limited only by the following claims.

What is claimed is:
 1. A dispersion managed single-mode opticalwaveguide fiber comprising:a core glass region, having a refractiveindex profile, surrounded by a clad glass layer, said clad layer havinga refractive index, n_(c), lower than at least a portion of therefractive index profile of said core glass region; said single-modewaveguide fiber having a varying total dispersion, which changes insign, from positive to negative and negative to positive, along thelength of said waveguide, wherein, sub-length, l_(i), of said waveguidefiber is made up of segments, dl_(i), each dl_(i) having an associated,essentially constant, total dispersion, D_(i), wherein D_(i) lies in afirst range of values of a pre-selected sign, and l_(i) is characterizedby the sum of products, D_(i) dl_(i), sub-length, l_(j), of saidwaveguide fiber is made up of segments, dl_(j), each dl_(j) having anassociated, essentially constant, total dispersion, D_(j), wherein D_(j)lies in a second range of values of sign opposite to that of D_(i), andl_(j) is characterized by the sum of products, D_(j) dl_(j), and,transition lengths l_(t) are provided over which the total dispersionchanges from a value in the first range of dispersion values to a valuein the second range of dispersion values, wherein the sum of all l_(i),all l_(j), and all l_(t) sub-lengths is equal to the waveguide fiberlength, and, the algebraic sum of all products dl_(i) D_(i) and dl_(j)D_(j) is less than a pre-selected value, over a predetermined wavelengthrange R.
 2. The single-mode waveguide fiber of claim 1 wherein saidtotal dispersions, D_(i) and D_(j), each have a magnitude in the rangeof about 0.5 to 20 ps/nm-km, the pre-determined wavelength range R isabout 1525 nm to 1565 nm, and the pre-selected value of the algebraicsum of products is essentially zero.
 3. The single-mode waveguide fiberof claim 1 wherein said sub-lengths, l_(i) and l_(j), are each greaterthan about 0.1 km.
 4. The single-mode waveguide fiber of claim 3 whereinany one of said transition lengths, l_(t), has a sub-length l_(s) lessthen about 500 meters over which the magnitude of total dispersion isless than about 0.5 ps/nm-km, thereby substantially minimizing powerpenalty due to four wave mixing over said sub-lengths.
 5. Thesingle-mode waveguide fiber of claim 4 wherein said core region has aradius, defined as the distance between the centerline of said waveguidefiber and the interface of said core region and said clad layer, and,wherein;said segment dl_(i) has an associated radius r_(i), whereinr_(i) is in a first pre-selected range, said segment dl_(j) has anassociated radius r_(j), wherein r_(j) is in a second pre-selectedrange, and, said transition length has a radius which changes from avalue, r_(i), in the first pre-selected range to a value, r_(j), in thesecond pre-selected range, and, wherein each r_(i) differs from eachr_(j) by an amount in the range of about 5% to 25%.
 6. The single-modewaveguide fiber of claim 4 wherein;said segment dl_(i), has a lightguiding region characterized by a maximum refractive index n_(i),wherein n_(i) lies within a first pre-selected range of refractive indexvalues, said segment dl_(j), has a light guiding region characterized bya maximum refractive index n_(j), and, the difference between each n_(i)and each n_(j) is at least about 5×10⁶.
 7. The single-mode waveguidefiber of claim 6 wherein the difference between each n_(i) and eachn_(j) is at least about 1×10⁻³.
 8. The single-mode optical waveguidefiber of claim 1 wherein said core glass region includes a centralportion, having a first index profile and at least one annular portion,adjacent said central portion, having a second index profile.
 9. Thesingle-mode optical waveguide fiber of claim 8 wherein said first indexprofile is an alpha profile.
 10. The single-mode optical waveguide fiberof claim 8 wherein said first index profile is constant andsubstantially equal to n_(c) and said adjacent profile has a roundedstep index shape and a maximum refractive index n₁ >n_(c).
 11. Thesingle-mode optical waveguide fiber of claim 10 wherein said adjacentprofile is an annulus, having an inner radius a₁ and an outer radius a₁said radii measured from the centerline of said waveguide fiber to theinner and outer edge, respectively, of said annulus, and a₁ /a is about0.5, and the maximum % index delta of said rounded step index is about1.0%.
 12. A dispersion managed single-mode optical waveguide fibercomprising:a core glass region, having a refractive index profile,surrounded by a clad glass layer, said clad layer having a refractiveindex, n_(c), lower than at least a portion of the refractive indexprofile of said core glass region; wherein a radius r is the distancefrom the waveguide fiber centerline to the interface of said core glassregion and said clad glass layer; said waveguide fiber has a lengthcomprising a number of sub-lengths, l_(j) ; l_(i) is made up of segmentsdl_(i), each dl_(i) having an associated, essentially constant,dispersion, D_(i), which lies in a first pre-selected range of values; anumber of sub-lengths, l_(j) ; l_(j) is made up of segments dl_(j), eachdl_(j) having an associated, essentially constant, total dispersion,D_(j), which lies in a second pre-selected range of values; and,transition lengths are provided between each sequential pair of l_(i)and l_(j), wherein; segments dl_(i) each have, a zero dispersionwavelength in a first wavelength range, segments dl_(j) each have a zerodispersion wavelength in a second wavelength range, said first andsecond wavelength ranges are disjoint, said second wavelength range islower than said first wavelength range, wherein, the algebraic sum ofdl_(i) * D_(i) and dl_(j) * D_(j) is equal to a pre-selected value, overa predetermined wavelength range R.
 13. The single-mode opticalwaveguide fiber of claim 12 wherein said first wavelength range is about1565 to 1680 nm and said second wavelength range is about 1480 to 1525nm.
 14. The single-mode optical waveguide fiber of claim 12 wherein themagnitude of total dispersion for any of said segments dl_(i) anddl_(j), is greater than about 0.5 ps/nm-km over an operating wavelengthrange which lies between the lower limit of said first range and theupper limit of said second range.
 15. The single-mode optical waveguidefiber of claim 14 wherein said operating range is about 1525 nm to 1565nm.
 16. A method of making a dispersion managed single-mode opticalwaveguide fiber comprising the steps:providing a glass core preformhaving a diameter and a length; reducing the diameter of at least onepre-selected sub-length of said core preform; overcladding said corepreform to yield a draw blank having a substantially uniform cylindricalshape; and, drawing said draw blank into a waveguide fiber having asubstantially uniform outer diameter, said fiber having at least onesub-length, l_(i), of reduced core diameter corresponding to the atleast one reduced core preform diameter sub-length, the remainder of thewaveguide fiber comprising sub-lengths, l_(j) of unreduced corediameter; wherein said at least one reduced core diameter sub-length ismade up of segments dl_(i), having an associated total dispersionproduct dl_(i) * D_(i), and said unreduced diameter sub-length is madeup of segments dl_(j), having an associated total dispersion productdl_(j) * D_(j) ; wherein the algebraic sum of products dl_(i) * D_(i)and dl_(j) * D_(j) is equal to a pre-selected value, over apredetermined wavelength range R.
 17. The method of claim 16 whereinsaid core preform diameter reducing step is accomplished by a techniqueselected from the group consisting of, acid etching and polishing,grinding and polishing, laser machining, and heating and stretching. 18.The method of claim 16 wherein the minimum sub-length of a segment ofreduced core diameter waveguide fiber is greater than 100 meters. 19.The method of claim 16 wherein the core preform reduced diameter isr_(i), which lies within a first range of values, and the core preformunreduced diameter is r_(j), which lies within a second range of values,and r_(j) differs from r_(i) by an amount in the range of about 5% to25%.
 20. The method of claim 18 wherein a plurality of reduced diametersub-lengths of said core preform are distributed uniformly along thelength of said core preform, the reduced diameter and unreduced diametersub-lengths being substantially equal in length, and the number of pairsof reduced and unreduced sub-lengths is an integer, n, and n≧0.
 21. Amethod of making a dispersion managed single-mode optical waveguidefiber comprising the steps:providing a core preform having a length, atleast one preselected sublength of said core preform having a relativelylarge diameter and at least one pre-selected sub-length of said corepreform having a smaller diameter than said relatively large diameter;overcladding said core preform to yield a draw blank, having asubstantially uniform cylindrical shape; and, drawing said draw blankinto a waveguide fiber having a substantially uniform outer diameter toproduce a waveguide fiber having at least one sub-length, l_(i), ofreduced core diameter corresponding to the at least one core preformsub-length of smaller diameter, the remainder of the waveguide fibercomprising at least one sub-length, l_(j) of core diameter larger thansaid reduced core diameter; wherein said at least one reduced corediameter sub-length is made up of segments dl_(i), having an associatedtotal dispersion product dl_(i) * D_(i), and said unreduced diametersub-length is made up of segments dl_(j), having an associated totaldispersion product dl_(j) * D_(j) ; wherein the algebraic sum ofproducts dl_(i) * D_(i) and dl_(j) * D_(j) is equal to a pre-selectedvalue, over a predetermined wavelength range R.
 22. The method of claim21 wherein the step of providing a core preform comprises providing apreform consisting entirely of core glass.
 23. The method of claim 21wherein said core preform has a step index profile.
 24. The method ofclaim 21 wherein the step of providing a core preform comprisesproviding a preform having a central core region surrounded by a layerof cladding glass.
 25. The method of claim 21 wherein the step ofovercladding comprises depositing glass particles on the surface of saidcore preform and sintering said particles to form a dense, clear glasscladding.
 26. The method of claim 25 wherein the step of sinteringresults in the formation of a glass cladding having a given diameterover the relatively large diameter portion of said core preform and adiameter smaller than said given diameter over the smaller diameterportion of said core preform, said method further comprising the step ofreducing the diameter of the portions of said cladding glass surroundingthe relatively large diameter portions of said core preform to form adraw blank having a substantially constant diameter cladding layer. 27.The method of claim 25 wherein the step of depositing glass particlescomprises depositing a layer of glass particles having a varyingdiameter which is greater over the smaller diameter regions of said corepreform, the diameter of the glass particle layer being such that, afterthe step of sintering, the diameter of the cladding layer of theresultant draw blank is substantially constant.
 28. The method of claim21 wherein the step of providing a core preform comprises the step ofheating at least one region of the core preform, and changing thediameter of the heated region.
 29. The method of claim 28 wherein thestep of changing the diameter of the heated region comprises traversingregions of the core preform adjacent the heated region toward the heatedregion to enlarge the diameter of the heated region.
 30. The method ofclaim 28 wherein the step of changing the diameter of the heated regioncomprises traversing regions of the core preform adjacent the heatedregion away from the heated region to reduce the diameter of the heatedregion.
 31. The method of claim 28 wherein the step of heating comprisesdirecting a flame onto said at least one region to heat said at leastone region and cooling portions of said core preform adjacent to said atleast one region.
 32. The method of claim 31 wherein the step of coolingcomprises directing a coolant gas onto those adjacent preform regions onopposite sides of the heated region.
 33. The method of claim 32 whereinthe step of heating comprises providing a burner having orifices fromwhich a combustion gas eminates, said combustion gas reacting to formsaid flame, said coolant gas emanating from orifices in said burneradjacent said combustion gas orifices.
 34. The method of claim 32wherein the step of heating comprises directing the flame from a burneronto said at least one region, said coolant gas originating fromorifices that are remote from said burner.
 35. The method of claim 28wherein the step of heating comprises directing the flame from a burneronto said at least one region, said burner being located at a firstazimuthal position with respect to the longitudinal axis of said corepreform, said at least one region being cooled by coolant gas streamsthat originate at an azimuthal position that is different than saidfirst azimuthal position.
 36. The method of claim 28 wherein the step ofheating comprises directing the flame from a burner onto said at leastone region, a portion of the flame being deflected by at least onebaffle.
 37. The method of claim 28 wherein the step of heating comprisesplacing heat conducting sleeves around the core preform on oppositesides of the region of the core preform that is to be heated, andheating the region of said core preform that is situated between saidsleeves.
 38. The method of claim 28 wherein, prior to the step ofheating, the diameter of any region of said core preform that is to beheated is reduced by removing material from the surface of said preform.39. The method of claim 38 wherein material is removed from the surfaceof said preform by a technique selected from grinding, etching and laserablation.
 40. The method of claim 28 wherein the step of heatingcomprises directing onto said at least one region a beam of laserradiation.
 41. The method of claim 40 wherein the power and duration ofthe laser radiation that is directed onto said at least one region issufficient to soften said region.
 42. The method of claim 40 wherein thepower and duration of the laser radiation that is directed onto said atleast one region is sufficient to ablate material from said region. 43.The method of claim 21 wherein the step of providing a core preformcomprises forming a preliminary core preform having a constant outsidediameter, feeding said preliminary core preform into the hot zone of adraw furnace to heat and soften a predetermined portion thereof, andpulling the softened portion of said core preform to stretch it andreduce its diameter.
 44. The method of claim 21 wherein the step ofproviding a core preform comprises forming a preliminary core preformhaving a constant outside diameter, feeding said preliminary corepreform into the hot zone of a draw furnace to heat and soften apredetermined portion thereof, pulling the softened portion of saidpreliminary core preform at a first rate to form to form said smallerdiameter sublengths, and pulling the softened portion of saidpreliminary core preform at a rate slower than said first rate to formto form sublengths of diameter larger than said smaller diametersublengths.
 45. The method of claim 44 wherein said preliminary corepreform has an aperture along its central longitudinal axis, whereinsaid method further comprising the step of evacuating said aperture, andwherein the steps of pulling cause said aperture to close.
 46. A methodof making a dispersion managed single-mode optical waveguide fibercomprising the steps:forming a draw blank having a length, at least onepreselected sublength of said draw blank having a relatively largediameter and at least one pre-selected sub-length of said draw blankhaving a smaller diameter than said relatively large diameter; drawingsaid draw blank into a waveguide fiber having a substantially uniformouter diameter to produce a waveguide fiber having at least onesub-length, l_(i), of reduced core diameter corresponding to the atleast one draw blank sub-length of relatively large diameter, theremainder of the waveguide fiber comprising at least one sub-length,l_(j) of core diameter larger than said reduced core diameter; whereinsaid at least one reduced core diameter sub-length is made up ofsegments dl_(i), having an associated total dispersion product dl_(i) *D_(i), and said unreduced diameter sub-length is made up of segmentsdl_(j), having an associated total dispersion product dl_(j) * D_(j) ;wherein the algebraic sum of products dl_(i) * D_(i) and dl_(j) * D_(j)is equal to a pre-selected value, over a pre-determined wavelength rangeR.
 47. The method of claim 46 wherein the step of forming a draw blankcomprises forming a preliminary draw blank having a uniform corediameter and a uniform cladding diameter, and reducing the diameter ofat least one pre-selected sub-length of said preliminary draw blank byremoving material from the surface of said cladding.
 48. The method ofclaim 47 wherein the step of reducing the diameter of said preliminarydraw blank is accomplished by a technique selected from the groupconsisting of, acid etching and polishing, grinding and polishing, lasermachining, and heating and stretching.
 49. The method of claim 46wherein the step of forming a draw blank comprises forming a uniformdiameter preliminary draw blank, and applying at least one claddingglass sleeve over said preliminary draw blank.
 50. The method of claim46 wherein the step of forming a draw blank comprises forming a uniformdiameter core preform, forming a periodically shaped cladding glasssleeve having at least one small diameter portion and at least one largediameter portion, and inserting said core preform into said sleeve. 51.The method of claim 46 wherein the step of forming a draw blankcomprises forming a uniform diameter core preform, forming uniformdiameter cladding glass sleeve, reducing the diameter of at least oneregion of sleeve to form periodically shaped cladding glass sleeve, andinserting said core preform into said periodically shaped sleeve. 52.The method of claim 46 wherein the step of forming a draw blankcomprises forming a uniform diameter core preform having a core regionsurrounded by cladding glass, forming spaced annular slots along surfaceof cladding glass, overcoating said core preform with cladding glassparticles, sintering said cladding glass particles to form a draw blankhaving a dense cladding glass layer, the length of said slots being lessthan 2 mm and being sufficiently short that the density of particlebuildup within said slots is sufficiently low compared to the density ofparticle buildup between slots that the cladding of said draw blank hasa relatively large diameter in those regions between said slots, theremainder of said cladding having a smaller diameter.
 53. A method ofmaking a dispersion managed single-mode optical waveguide fibercomprising the steps:providing a cylindrical core preform having adiameter and a length; overcladding said core preform to yield a drawblank, having a uniform cylindrical shape; reducing the diameter of saiddraw blank over at least one pre-selected sub-length; and, drawing saiddraw blank into a waveguide fiber having a uniform outer diameter, andhaving a sub-length of reduced core diameter corresponding to thereduced diameter sub-length of said draw blank; wherein, said at leastone reduced core diameter waveguide sub-length l_(i) is made up ofsegments dl_(i), having an associated product dl_(i) * D_(i), and theunreduced core diameter waveguide sub-length l_(j) is made up ofsegments dl_(j), having an associated product, dl_(j) * D_(j) ; whereinthe algebraic sum of products dl_(i) * D_(i) and dl_(j) * D_(j) is equalto a pre-selected value over a pre-determined wavelength range R.
 54. Amethod of making a dispersion managed single-mode optical waveguidefiber comprising the steps:providing a draw blank, having asubstantially cylindrical core portion and a surrounding clad layer,having a substantially cylindrical outer surface; drawing waveguidefiber from said draw blank, using waveguide fiber drawing apparatus,said apparatus including a furnace and, spaced apart from said furnace,means for applying a polymer coating to said waveguide fiber; and, aftersaid waveguide fiber leaves the furnace and before said waveguide fiberreceives a polymer coating, irradiating said waveguide fiber withradiation over pre-selected and spaced apart longitudinal segments,l_(r), wherein l_(r) is made up of segments dl_(r) having essentiallyconstant total dispersion D_(r), and l_(u) are the lengths of waveguidefiber which are not irradiated, wherein l_(u) are made up of segmentsdl_(u) having essentially constant total dispersion D_(u) ; wherein thealgebraic sum of products dl_(r) * D_(r) and dl_(u) * D_(u) is equal toa pre-selected value over a pre-determined wavelength range R.
 55. Themethod of claim 22 wherein said radiation is selected from the groupconsisting of ultraviolet, gamma ray, x-ray, beta particle and alphaparticle, and neutron radiation.
 56. The method of claim 23 wherein saidradiation is ultraviolet radiation which is incident upon saidlongitudinal segments and is characterized by an energy flux of about100 mJ/cm² delivered as a pulse of time duration in the range of about10 to 20 nanoseconds, wherein at least one pulse is delivered to eachsaid longitudinal segment.